The technical realization of reading out a memory, for example a non-volatile memory, necessitates solving several technical problems. There are several criteria to be met.
For one thing it is important that a circuit suitable for reading out the memory be designed in the cost-effective manner. This means that the silicon area necessary for the realization of the circuit is to be kept small. Furthermore, it is necessary to read out the cell as fast as possible at low current consumption. The access time for the data of the non-volatile memory depends on the specification of the package containing the non-volatile memory. The package may for example be a chip card. In a chip card, the access time of the card is substantially determined by the time of reading out the memory cell itself. Here, a NOR flash stands out compared to a NAND flash by means of its faster access time. Moreover, for a total chip containing a non-volatile memory, typically, a maximum current is prescribed in the specification. In addition, and in particular for a contactless chip card, maximum permitted current peaks are prescribed in the specification. The requirements given by the specification are broken down to all circuit parts and therefore also to a sense amplifier necessitated for reading out the memory. Here, the sense amplifier deserves special attention, as it substantially contributes to the peak current because of its frequency of occurrence.
For reading out the memory, an adaptive current readout method may be employed. Specifically in a non-volatile memory, information to be stored may be stored in the form of a starting voltage variation of a selection transistor. The starting voltage varying of for example an N-FET also means, however, that an “ON” and “OFF” current of the selection transistor changes or defines at the same enabling voltage. As in a memory consisting of several memory cells, only ever one memory source per bit line or sense line, respectively, is enabled, and all other cells of the same bit line or sense line are non-conducting, that is “OFF”, the “OFF” current is dependent on the programming of the cells also connected to the bit line. Reading out a cell signal is dependent on the programming of the cells not to be read out because of the “OFF” current of the cells not to be read out. This factor results in the necessity of adaptive reading out. This means that the states, that is the current flows, of the memory cells before and after the enabling must be compared to each other. For example, the “OFF” current prior to the enabling may be compared to the assumed “ON” current after the enabling. This means that, at a given word line voltage, opening the transistor will not take place and the resistor therefore gates off (is turned off). If the current has substantially increased after the enabling, then the enabling voltage will have sufficed to open the cell in the form of the selection transistor. Therefore, the starting voltage of the cell is lower than the word line voltage and/or the enabling voltage. In order to retrieve the memory information stored in the memory cell, the “OFF” current is stored by means of a programmable current source prior to the enabling and compared to the “ON” current after the enabling.